(1) Field of the Invention
The present invention pertains to probes for monitoring corrosion. Particularly for monitoring corrosion within pipes or tanks by utilizing electromagnetic radiation such as light.
(2) Background of the Invention
Corrosion presents a very real problem in systems which store, use, or transport corrosive materials. It is particularly important in piping or storage systems where corrosive materials, such as liquids or gases being stored or transported, may be in contact with the interior surface of a pipe or tank for an extended period of time, and where damage from corrosion may not be readily visible. If ignored, corrosion can eventually damage the piping or tank to a point where leaks or failures occur resulting in significant repair costs and possible other dangers.
While corrosion can occur in any type of piping or tank, it is particularly problematic in underground, undersea, or other difficult to access areas. Pipes susceptible to corrosion can be carrying crude oil, water, steam, or any other fluids or gases which are known to be corrosive to the metals of which piping is usually constructed. These pipes can also be corroded by the materials (such as water or earth) that they are placed within. Further, failure of these pipes can result in danger to occupants of buildings, can result in contamination of groundwater or similar resources, or can cause environmental damage.
Two methods are typically used to reduce and/or control corrosion in piping and storage systems: The first of these is passivity of the material surfaces. This type of control generally is used in the selection of materials for the piping or which coat the piping. These materials are generally chosen to be relatively inert compared to the fluids housed thereby and any surrounding materials. While this type of control can be effective, it is often very expensive. Further, coatings will often mechanically wear in a flow or may develop cracks over time which can lead to failure. Further, there are relatively few materials which are able to resist the wide variety of potentially reactive materials which may be in a process stream.
Because material selection is necessarily imperfect and quite expensive, the most common method for attempting to control corrosion is through the addition of corrosion inhibiting or control chemicals to the process stream which is within the pipe or tank. This method also is quite expensive due to maintenance down cycles and the ongoing costs associated with the required chemicals. Further, the method can actually create additional problems as the corrosion inhibiting chemicals may need to later be filtered out and may prove damaging to other systems which come into contact with the process stream. This method, however, often provides for controlled levels of inhibition at reasonable cost.
In many respects, corrosion control is not really the activity of trying to prevent corrosion outright, its simply trying to inhibit corrosion as much as possible so as to allow for piping and other products to have the longest possible useful life. Therefore, that corrosion occurs is often inevitable, the desire is simply to understand how much corrosion is occurring, attempting to reduce this amount of corrosion in a cost effective way, and being able to estimate and plan for maintenance and replacement costs.
It is well recognized that knowledge of the corrosion process that is occurring in the tank or pipe can help alleviate or reduce the continuing costs of unnecessary maintenance cycles and provide more cost effective use of chemical additives or passive materials. In the simplest sense, most process fluids will be comprised of a myriad of materials which may change over time. Therefore, the ability to actively monitor what corrosion is occurring provides for the most effective ability to react to these changes. Additionally, monitoring allows the operator to schedule maintenance as required due to actual system performance instead of on a fixed schedule, which may not be sufficient to eliminate failures or which may be overly conservative and, therefore, not cost effective. Therefore, an important component of corrosion resistance is detection of the type and rate of corrosion occurrence. With accurate detection, preventative strategies, such as inhibiting chemicals, can be added more efficiently.
Characterization of the corrosion process is preferred in the current production environment in order to meet increased requirements for profitability, worker safety, and environmental responsibility. Further, it provides for an improved ability to correctly react to a change in corrosion characteristics. With accurate determinations of corrosion, minimum amounts of chemical additives useful to hinder that type of corrosion may be added to the process streams.
It is well recognized that the most effective strategies for corrosion monitoring are placed within the pipe or tank so as to interact directly with the process stream. While external monitoring systems can be used, these systems often lack the sensitivity of systems placed in direct contact with the corrosion causing materials. Further, systems within a piping system or tank can more easily measure corrosion occurring in fairly static arrangements, such as a pipe system where water is maintained in a steady pressurized state as opposed to flowing.
The most prevalent monitoring method currently used is a coupon test system, where various coupons (typically materials used in the construction of the piping or storage system or which are particularly good indicators of corrosion) are placed within the system and exposed to the process stream. These coupons then corrode, or show effects indicative of corrosion, in a manner which allows extrapolation of the corrosion within the entire piping or storage system. While this method works reasonably well, it often requires draining of the piping or storage system to remove the coupon for inspection and analysis. Furthermore, coupons can be difficult to use in inaccessible locations such as underwater, underground, or in dangerous environments as they are not easily retrieved. Coupons also are not necessarily exposed to the same forces as the pipe walls due to required positioning. Coupons also cannot give real time determinations of the corrosion within the piping or storage system, so conditions can only be determined during scheduled maintenance periods, when the coupons can be removed or analyzed. This means an unexpected problem can develop within the maintenance interval without warning. Further, these systems are not very reactive to sudden changes.
There have been designs proposed for real time corrosion probes which measure electrical properties of the process stream, or which are able to measure various other properties of the process stream directly. The problem with these systems is that many process streams do not conduct electricity rendering the first type of probes unusable for these applications. Secondly, often a particular characteristic of a process stream can provide nothing more than an educated guess as to how mush corrosion is actually occurring. In these situations, a large amount of data may need to be collected and analyzed for the determination, and even large collection systems may not take into account every factor that needs to be examined to accurately measure corrosion. Further, many corrosive process streams are also corrosive to probes or measurement devices in the probes resulting in their destruction over time. This can lead to frequent replacement of expensive probes and increases the possibility of probe malfunction due to damage.